A mong the endless procession of motorists and pedestrians who pass by the gently faded 1920s family home on Scraptoft Lane each day on their way to and from the centre of Leicester, few realise they are so close to one of the most important wildlife sites in the British Isles. Fewer still will be conscious that the elderly woman in the wheelchair who they might occasionally glimpse working at her desk in the light of the bay window is one of the great heroines of the 20th-century environmental movement.

For here in the heart of the English Midlands, amid the utterly ordinary surroundings of an everyday suburban garden, Jennifer Owen, now aged 75, spent three decades observing an extraordinary habitat. Over the course of those years, during which she raised a family and enjoyed a successful academic career, the now-retired university lecturer identified 2,673 different species living in her "neat, productive" plot. Among the daisies, buddleia and orderly lines of lettuce there were 474 plants, 1,997 insects, 138 other invertebrates such as spiders and woodlice and 64 vertebrates including 54 species of birds and seven mammals. It is estimated that had she the time and expertise available, the final tally would have been in excess of 8,000 species. And all of this within a patch of land measuring no more than 741sq m.

This month the full findings of her life's endeavours are to be published by the Royal Horticultural Society. But while Wildlife of a Garden: A Thirty-Year Study – the follow up to her 1991 The Ecology of a Garden – is a timely reminder that a fascinating natural world flourishes beyond the savannas, jungles and wildernesses we see on our television sets, it also makes sober reading for nature lovers. For despite the proliferation of back garden life, she found the number of creatures has been remorselessly declining since the project began in 1972.

Yet what makes the story of her study so inspiring is not just that it was completed with an indomitable spirit which meant for the last 26 years the naturalist was battling multiple sclerosis, it is that it was done without any official funding. Though yielding countless scientific papers and articles, it was entirely dependent on the goodwill of fellow experts in universities and research centres in Britain and America. Today, tragically, Mrs Owen's illness means she is rarely able even to get into the garden in which she spent so many happy hours. Since she made her last records in 2001, the plot has endured the fate of so many of its neighbours. Once home to broad leaf trees, vegetables and a pond, it has been drained, felled and block-paved to provide wheelchair access and homes for cars as well as outside seating areas. But life still thrives there – should anyone choose to look for it.

"Gardens are extraordinarily rich habitats," she explains, sitting surrounded by the books and mementoes of a life of irrepressible intellectual curiosity. In the area of parasitic wasps (Ichneumonidae) alone , she discovered six species previously unknown to science. The project began long before the phrase organic gardening had been coined. "I didn't use pesticide or poison on any creature because I was interested in what shared the garden with me. I enjoyed it and I was not a vicious or overly tidy gardener, though my father, who was very much of the old school, used to walk around with his eyes averted," she says.

The Owens' front and back gardens were "quite a jungle" when she and her husband Denis, also an eminent zoologist, moved in after paying £9,000 for the four-bedroom home. At first they began hand netting and recording butterflies with the help of their children. They also deployed a Malaise trap – a large open-sided tent in which specimens drop into a collecting jar. A pitfall trap, sunk into the ground, was also used to harvest beetles and centipedes while a light attracted moths. Despite being an expert in the field of hoverflies, she found she was unable to identify many of the other species herself – especially after her husband moved to a new teaching post at Oxford in the early years of the study. Each Sunday night between 1 April and 31 October, she would empty the traps and sort their contents into groups at the kitchen table alone before sending them to obliging colleagues for identification.

Mrs Owen's realisation of the sheer variety of life forms right under our noses came while teaching in Africa in the 1960s. "My garden had more species of butterflies than the nearby rainforest which is a rich habitat by any standards. The explanation was that we got the savanna species as well. So coming back to Leicester it was automatic. Once you start looking at a habitat in some detail and start collecting records it becomes an exponential process. The more you do, the more valuable the records become because there is more to go on, so you find out more and more," she says.

It "never occurred" to her, she says, that her observations would one day become a nationally or even internationally important record charting the impact of climate change and suburban habitat loss in the latter years of the last century although she admits with some pride that she was once the subject of a question on Stephen Fry's QI – a fact which seriously impressed her grandchildren.

Nor indeed did she see anything out of the ordinary in keeping her meticulous records. "I used to love working in my garden, sowing seeds and taking cuttings. You never get blasé because there is always something odd turning up. In 1987 I suddenly noticed an enormous black and white banded hoverfly that was nothing like I had ever seen before. It was a species that lives in northern moorland and here it was in my back garden wandering around looking for something to eat," she says. "The drought year of 1976 was interesting in many ways. The butterflies were so hard pressed to find nectar because all the flowers had dried up, forcing so many species to wander far from home." The 1970s were indeed a boom time for insect and garden life, the study shows. The weather was more reliable and British farmers' quest for productivity had yet to reach its most chemically destructive. Since then she has recorded a decline in the number of moths, butterflies, hoverflies, ladybirds and bees, although beetles and solitary wasps have bucked the trend. "If you get a really good summer then the numbers go up but they have never recovered to the level of the 1970s," she adds.

Yet she refuses to slip into pessimism, believing initiatives such as government stewardship schemes and the endeavours of individual gardeners are reason to remain cheerful. "A lot of things are bad but you have to realise where things are good and where there is hope and to capitalise on that. We don't even know how many species there are in the world. We probably only recognise one tenth but because of environmental changes more species are becoming extinct all the time. It is woeful and tragic; I feel rueful at people's blindness and stupidity but I can't do anything about it," she says.

Mrs Owen grew up in Leicester. Her father, a textile factory owner, was one of the country's leading amateur ornithologists and her interest in wildlife flourished working alongside him catching, ringing and recording the birds that visited their large garden. She studied zoology at Oxford University under some of the towering academic figures of the day such as Charles Elton, and it was here she met her husband Denis Owen, who later became a well-known broadcaster, writer and academic. "They were the happiest days of my life," she recalls. "I partied but I also got a First."

After graduation the couple travelled to the US, Uganda, Sierra Leone and Sweden, before the Scandinavian winter persuaded her to return to her home city with her two children. The Leicester garden became a place for neighbours or local naturalists seek help, although not always with happy consequences. She recalls one occasion when a man brought what he believed to be a rare beetle to her door only to be informed it was a tick, engorged with the blood of his pet dog. "He threw it in the gutter and ran it over with his car. He was absolutely horrified," she chuckles.

Mrs Owen is convinced that the invertebrate world is too often overlooked in favour of the"cuddly animals" beloved of teatime viewers. "Invertebrates are often much more interesting because there is so much fascinating detail to them. They are all about you in your garden and you should make the most of them. It is always the fur and feather brigade which are regarded as acceptable although I suppose butterflies get to be honorary members and are allowed in," she says.

As for a legacy, she hopes her work might inspire others. "I would like to make people more aware of how much fascination and interest there is in their garden and for them to be seen as valuable habitats. There is now an incredibly well-informed population of people who read and watch good television and people are much more open-minded and appreciative than they were 20 to 30 years ago. But I haven't written this book in a missionary sense. Having done so many years of work and having produced some results that I think are interesting I jolly well want them to be known."

On record From Jennifer Owen's book

Plants

A gardener crowds together in one place a far greater diversity of plants than is ever found in one place in the wild. Even in tropical rainforest an area equivalent to a typical garden would not contain so many different plant species. Some 412 plant species in total grew in the garden between 1975 and 2001, 311 that were cultivated, and 101 that came in of their own accord; in 1984, for instance, my garden contained 264 species of flowering plant, excluding grasses, 197 of them intentionally cultivated, in a planted area of 741m2, a species diversity of 3,563 species per hectare.

It could be argued that most gardeners grow more or less the same range of species, so the floristic composition of one garden is repeated again and again, making the average species diversity per hectare smaller, the larger the area considered. True, the plant diversity of one garden may be similar to that of other gardens in the same area of suburbia, but not identical, for different gardeners grow different species and cultivars and manage their land in different ways.

Bees

Think of a rich and productive garden and you think of one alive with the diligent activity of bees at flowers, the air vibrant with their busy humming. In my garden, the mauve, hooded flowers of spotted dead-nettle (Lamium maculatum) dip and sway with the attentions of visiting bumblebees, from early in spring to late in the autumn, the tall violet-blue stems of sage flowers (Salvia officinalis) shimmer with their comings and goings, and the trumpet-like hollyhock flowers (Alcea rosea) seem to amplify the buzz of the fat, furry visitors.

The bumblebees so conspicuously active at flowers and the yellow wasps that disrupt picnics are social species. Colonies are founded in spring by overwintered queens, most of whose offspring are non-breeding female workers that remain associated with the nest, building cells, tending the queen's eggs and larvae, defending the colony and foraging for food. Social species are the commonest species of bees and wasps in the garden, but Malaise trapping has revealed an unexpected diversity of less-conspicuous solitary species.

In total, 59 species of bees and 62 of wasp were recorded in the Leicester garden during the period 1975-2001.

Beetles

Beetles are not usually conspicuous but the gardener may be aware of several types. Shiny, black ground beetles lurk beneath vegetation and stones; small flea beetles, yellowish-brown or black and yellow, are often abundant; tiny, shiny or metallic, dark beetles are common in flowers; bright ladybirds attract attention and they and their prehistoric-looking larvae are often amongst aphids. There are more species of beetles in the world than of any other type of animal. They are not, however, as amenable to trapping as some insects, and the 422 species I have recorded probably represents only a fraction.

Insects

The dominant group in my garden, and indeed in all gardens, is the insects. They are the most conspicuous group of invertebrates, although many other invertebrate groups occur, and some of those in which individual size is small or microscopic, such as mites, are undoubtedly very abundant. Several types of larger invertebrates, particularly spiders, were collected and identified, providing species lists. Furthermore, myriapoda (Chilopoda and Diplopoda) and harvestmen (Arachnida – Opiliones) that fell into pitfall traps, were identified, providing comparable annual samples for the 11 years from 1980–1990.

A visit to the garden by AJ Rundle, as part of a survey of Leicester's terrestrial invertebrates that he undertook for the Leicestershire Museums, Arts and Records Service, resulted in many additions to the garden species list. Over the years, an extensive species list of invertebrates, other than insects, was compiled.

Mammals

A wood mouse was seen in the backyard and on the patio on numerous occasions, particularly in the late 1990s. This is the most abundant mammal in the city and county, being even commoner than the house mouse.

The house mouse is common and widespread in the county, although probably under-recorded. Foxes were not seen in the garden until 1992, although they had often been smelled before then. Since then, one or two foxes have regularly been seen. They became established in many British towns and cities in the inter-war years when there was a boom in building low-density housing with medium-sized gardens (Harris & Rayner, 1986). They are common and widespread in the county of Leicestershire, and since the 1980s have been regularly seen in the city, some garden-owners having resident foxes, which they supply with food.

Dogs rarely entered the garden, but cats were frequent, and constantly had to be chased away as they posed a threat to the garden's birds. The 70 domestic cats of a Bedfordshire village of 173 houses were judged in 1981-1982 to be major predators, 1,090 prey items (22 species of birds and 15 of mammals) being taken in a one-year period, the chief being wood mice (17 per cent), house sparrows (16 per cent) and field voles (14 per cent) (Churcher & Lawton 1987).

Moles are common in the city, but in many gardens, the soil is disturbed too much by digging for them to feel at home. Muntjac, small non-native deer, are occasionally reported, and one was seen in Humberstone nearby, but the position of my garden makes it unlikely that one will visit.

Birds

Sparrowhawk, kestrel, red-legged partridge, turtle dove, tawny owl, lesser spotted woodpecker, fieldfare, redwing, chiffchaff, blackcap, marsh tit, nuthatch, treecreeper and jackdaw were irregular visitors to the garden. Sparrowhawks were not seen until 1987, but thereafter were sighted in the garden on several occasions, often carrying carcasses of birds. Kestrels were repeatedly seen, occasionally perching on the television aerial or garage roof, and on one occasion, hunting low over the garden. Red-legged partridges were twice disturbed from vegetable patches where there had been previously unexplained damage to vegetables. A single turtle dove was seen on two occasions in the 1990s, on one occasion eating elderberries.

The total garden bird list is 54 species, which could be boosted to 59 if racing pigeons and escaped cage-birds (three species of parrot and a cockatiel) are included. A larger garden with greater structural diversity, near the coast or a large body of inland water, would have a far longer list, but the list is impressive for a relatively small garden.

Wildlife of a Garden: a Thirty-year Study, by Jennifer Owen RHS, 15 November 2010 RRP £30. Independent readers can buy the book at a special price of £25 (until 31 December). Call RHS mail order, 0845 260 4505 or visit www.rhsshop.co.uk quoting INDEWILD.